Recently, demand for high-strength automotive parts has markedly increased to allow for reductions in the weight of automobiles for low exhaust gas emissions and good gas mileage as well as to improve the impact safety of automobiles. To this end, high-ductility, high-strength steel sheets have been developed and commercialized.
However, the weight of automobiles may increase due to more parts being used to improve safety and convenience, while relatively heavy batteries are used in next-generation electric automobiles using electrical energy instead of petroleum. However, there is a limit to decreasing the weight of automobiles through the use of the above-mentioned high-ductility, high-strength steel sheets. Therefore, it is necessary to use lightweight materials such as steel sheets having a low specific gravity to additionally decrease the weight of automobiles. Such steel sheets having low specific gravity have very high degrees of strength and ductility but are relatively inexpensive as compared with a rival material, aluminum (Al), and are thus considered as a substitute that can satisfy market demand.
Advanced high strength steels (AHSS) such as dual phase (DP) steels and transformation induced plasticity (TRIP) steels are currently typical of high-ductility, high-strength steels used for automobiles. However, since strength and ductility are obtained in such steels through including martensite or retained austenite in ferrite, such steels are subject to deformation by the mechanism of dislocation glide and are limited to having good ductility at high strength due to interfaces between different types of microstructure.
A typical technique for another kind of steel is disclosed in Korean Patent Application Laid-open Publication No.: 1994-0002370: high-strength twinning induced plasticity (TWIP) steel including 15% or more of Mn. The TWIP steel has a single-phase austenite microstructure and highly improved strength-ductility characteristics but has low yield strength at a yield ratio of 40% to 60%, and is thus difficult to be provided with sufficient rigidity for automotive structural parts. Furthermore, the addition of a large amount of Mn may increase manufacturing costs and decrease the productivity of production lines.
In addition, high-strength, high-ductility steel sheets having a low specific gravity, owing to the addition of a light element, Al, have been developed. A typical technique therefor is disclosed in European Patent No.: EP0889144. The disclosed technique relates to an austenitic steel sheet formed by adding 8% or less of Al and 10% to 30% of Mn to low carbon steel. Since the disclosed steel sheet has a low carbon content, a large amount of Mn is added to stabilize austenite therein. However, when the disclosed sheet is deformed, austenite may easily be transformed into martensite to thereby decrease ductility.
Japanese Patent Application Laid-open Publication No.: 2006-176843 discloses steel including 0.8% to 1.2% of carbon, 10% to 30% of Mn, and 8% to 12% of Al. Since the content of Mn is low, with respect to the content of Al, large amounts of precipitates such as (Fe,Mn)3AlC are present, which decreases ductility and facilitates delayed fractures caused by hydrogen absorption after processing.
An aspect of the present invention provides an austenitic, lightweight, high-strength steel sheet including appropriate concentrations of alloying elements such as Mn, Si, and Al to obtain a tensile strength of 800 MPa or greater, an elongation of 30% or greater, and a yield ratio of 60% or greater that are difficult to obtain from high-carbon, high-manganese steel sheets or high-manganese, lightweight steel sheets of the related art.
According to an aspect of the present invention, there is provided an austenitic, lightweight, high-strength steel sheet having a high yield ratio and ductility, the steel sheet including, by weight %, C: 0.6% to 1.0%, Si: 0.1% to 2.5%, Mn: 10% to 15%, P: 0.02% or less, S: 0.015% or less, Al: 5% to 8%, Ti: 0.01% to 0.20%, N: 0.02% or less, and the balance of Fe and inevitable impurities, wherein the steel sheet has a specific gravity of 7.4 g/cm3 and a Mn/Al ratio of 2 to 3.
The steel sheet may be one of a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet. The steel sheet may further include at least one selected from the group consisting of Cr: 0.1% to 3.0%, Ni: 0.05% to 2.0%, Cu: 0.1% to 2.0%, and Mo: 0.05% to 0.5%. The steel sheet may further include at least one selected from the group consisting of V: 0.005% to 0.5%, Nb: 0.005% to 0.2%, Zr: 0.005% to 0.2%, and B: 0.0005% to 0.0030%. The steel sheet may further include one or two selected from the group consisting of Sb: 0.005% to 0.2% and Ca: 0.001% to 0.02%. The steel sheet may have a single-phase austenite microstructure. The steel sheet may have a tensile strength of 800 MPa to 1200 MPa, a yield ratio of 60% or more, and elongation of 30% or more.
According to another aspect of the present invention, there is provided a method for producing an austenitic, lightweight, high-strength steel sheet having a high yield ratio and ductility, the method including: hot rolling a slab at a hot-rolling start temperature of 1000° C. to 1200° C. and a hot-rolling finish temperature of 850° C. or higher so as to form a steel sheet, wherein the slab includes, by weight %, C: 0.6% to 1.0%, Si: 0.1% to 2.5%, Mn: 10% to 15%, P: 0.02% or less, S: 0.015% or less, Al: 5% to 8%, Ti: 0.01% to 0.20%, N: 0.02% or less, and the balance of Fe and inevitable impurities, and the slab has a specific gravity of 7.4 g/cm3 and a Mn/Al ratio of 2 to 3; and coiling the hot-rolled steel sheet at a temperature of 600° C. or lower.
Prior to the hot rolling of the slab, the method may further include cooling the slab and reheating the slab to a temperature of 1000° C. to 1200° C. After the coiling of the hot-rolled steel sheet, the method may further include: cold rolling the coiled steel sheet at a reduction ratio of 20% to 70%; after heating the cold-rolled steel sheet the cold-rolled steel sheet at a rate of 1° C./s to 50° C./s to a temperature equal to or higher than a recrystallization temperature but not higher than 900° C., annealing the cold rolled steel sheet for 10 seconds to 180 seconds; and cooling the annealed steel sheet at a rate of 1° C./s to 100° C./s. The method may further include plating the steel sheet with at least one selected from the group consisting of Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si at a plating density of 20 g/m2 to 120 g/m2.
The present invention provides a steel sheet having a low specific gravity, a high yield ratio, and high ductility as compared to high-strength steel sheets of the related art such as advanced high strength steels (AHSS). The steel sheet of the present invention is effective in maintaining the rigidity of a structural member and have good press processing characteristics so that the steel sheet can be used for manufacturing automotive parts to reduce the weight of automobiles and combining a plurality of parts into a single module or as one part to simplify machining or assembly processes.